Wire supported bicycle wheels consist of a hub supported by spokes of known shape within a circular rim. As the wheel rotates, the spokes are continuously subjected to compressive (at the bottom) and tensile (at the top) stresses. These cyclical stresses cause problems both in the structural portion of the spoke as well as in the fittings which attach the spoke to the rim and hub. It is a purpose of this invention to design a wheel system which minimizes the affect of these cyclical stresses.
The wheel, taken as system, must be designed to balance weight reduction with strength. It is a purpose of this invention to combine the highest strength materials in a system having low weight, with a primary goal of minimizing the rotational mass of the wheel. An essential part of this goal includes a method of manufacturing the wheel system which is inexpensive and efficient.
The assembly of spokes within the wheel system forms the critical element of the support structure for the wheel and is primarily responsible for the strength and performance characteristics of the wheel. The most efficient method of distributing forces within the spoke assembly is to assemble the wheel components in a manner that exerts the forces on the spoke body in a straight axially aligned pull without any bends or angles in the spoke. In addition the method of assembly must avoid winding or twisting the spoke along its length. It is a purpose of this invention to connect the spokes to the rim and hub by means of fittings which facilitate an axially alignment of the forces and avoids inducing torsional stresses.
U.S. Pat. No. 4,729,605 represents an attempt at accomplishing weight reduction by using fiber reinforced rods as the spokes. Chopped carbon fibers are mixed with a binder such as epoxy or aluminum to form a matrix. The matrix is formed into a spoke to which is attached a fitting at either end. The fitting which connects the spoke to the rim is threaded to enable adjustment of the tension. This is a rigid member with high cut strength and provides a weight reduction of between 3 and 4 grams per wheel. It is a purpose of this invention to reduce the weight of the wheel further while increasing the tensile strength of the spokes and lowering the mass of the rim. It is important to avoid abrasion and stress failures caused by the brittle nature of resin based fiber materials.
A non-rigid spoke is described in U.S. Pat. No. 5,110,190 which employs cylindrically or ribbon bundled aramid fibers such as KEVLAR.RTM. to form a spoke having fittings at either end. Although these fibers exhibit lower creep than some fibers, creep is nevertheless present. Creep causes a gradual elongation of the fibers which, in many ways, defeats any advantage gained by the non-rigid feature of the spoke of the '190 patent. The spokes will lose tension, the wheel will distort and realignment and truing will be required on a frequent basis. It is extremely difficult to maintain consistent tension on such spokes because of the affect of creep. It is a purpose of this invention to minimize creep while allowing a slight stretching of the fibers.
In addition the '190 patent ignores the important contribution of the fittings used to attach the spoke body to the rim and hub. The connections of spoke to rim and hub do not allow for an axial alignment of the forces at the point of connections and will cause the fibers to abrade against adjacent fibers under the cyclic stresses. The '190 patent also fails to recognize that aramid fibers deteriorate under exposure to ultra violet radiation. It is a purpose of this invention to minimize the effect of environmental influences on the spoke body.